bitwise operations

Question

// PWM frequency:
// 0 - 48 kHz
// 1 - 12 kHz
// 2 -  3 kHz
enum { MOTOR_FREQUENCY = 1 };

// Configure Timer 2 w. 250x period.
T2CON = 1 << 2 | MOTOR_FREQUENCY /* << 0 */;

Have i understood this right?

11111111 Arithmetic left-shift-by-two of 0 or 1 or 2

Means:

T2CON = 1 << 2 | 0 = 1111 1100    
T2CON = 1 << 2 | 1 = 1111 1000    
T2CON = 1 << 2 | 2 = 1111 0000

Kind Regards, Sonite

Answer 1

Context:

TCON2 is a timer register on PIC MCUs, where the last two bits configure the prescaler.

• T2CKPS[1:0] = 0b00 = 0 => /1 prescaler
• T2CKPS[1:0] = 0b01 = 1 => /4 prescaler
• T2CKPS[1:0] = 0b1x = 2 or 3 => /16 prescaler

Bit 2 actually switches the timer on, so it always needs to be set to do anything, hence the 1 << 2 (which really should be written as 1 << T2CON_TMR2ON_bit with T2CON_TMR2ON_bit being defined in some CPU-configuration header)

All said and done, the three settings are 0b100, 0b101, and 0b110, which turn on the timer, and tweak the prescaler to get those frequencies mentioned in the comments.

Also, using an enum with one element is just about pointless; use #define.

Answer 2

1 << 2 = 100b

So with the OR:

100b | 1 = 101b
100b | 2 = 110b

Answer 3

Assuming that you are playing with a microcontroller with 8-bit registers .

0000 0001 << 2 = 0000 0100
then 
0000 0100 OR 0000 0000 = 0000 0100
-----
0000 0001 << 2 = 0000 0100
then 
0000 0100 OR 0000 0001 = 0000 0101
-----
0000 0001 << 2 = 0000 0100
then 
0000 0100 OR 0000 0010 = 0000 0110